It all starts with defining what morality means. The way I would define morality is behaviors that maximize flourishing for sentient creatures and minimize suffering. While it is clearly difficult to quantify flourishing and suffering, there are behaviors that clearly cause suffering in this world and impede the opportunity for flourishing and, by the above definition of morality, are plainly immoral. The way I see it, rejecting the possibility that flourishing and suffering can be quantified at all is the only argument that can be made against moral absolutism. Everything else is just quibbling over relevant variables across the spectrum of available behaviors to determine what makes them more or less moral. There is always a behavior that is objectively the more moral choice, but it might be difficult in practice to determine which is the more moral choice due to a lack of available relevant data. The absence of said data shouldn't be assumed to be because it doesn't/can't exist, but rather that it hasn't been collected. Rejecting the idea that there is always a more moral behavior amongst several choices is the dangerous assumption, imo.